Recent Advances in Ionospheric Stimulated Electromagnetic Emission Investigations (Invited Paper)

Stimulated Electromagnetic Emission (SEE) is secondary radiation produced during experiments where high power high frequency (HF) radio waves interact with the ionosphere. SEE has been a vibrant area of investigation for the past three decades and SEE has been developed into an important remote sensing tool for the ionosphere with substantial potential for aeronomy studies. Past experiments primarily considered emissions within a bandwidth of 100 kHz of the pump (transmit) frequency, so-called wideband SEE (WSEE), but more recent work has concentrated on SEE within 1kHz of the pump frequency, so-called narrowband SEE (NSEE), as well as the pump frequency harmonic. A brief review of the recent experimental observations of NSEE are presented as well as some basic concepts of the nonlinear wave-wave processes that produce NSEE. Some new potential applications are discussed as well.